1. Field of the Invention
The present invention is in the field of thermal (infrared) target simulators used in testing imaging systems.
2. Description of the Prior Art
In order to test thermal target viewers, imagers, and the like, simulators must be used. In addition, a target which can be used to characterize the thermal image degradation due to atmospheric propagation of the image radiation field from close to the target to the thermal image at the imager acquisition range is required. In the area of target acquisition the visible is the most convenient because the human eye has adapted to distinguishing objects by the dominant source of reflected spectral radiation, the sun a 6000 degree K blackbody radiator whose radiation peak is between 0.4 and 0.7 microns. The sun is not always present to illuminate the target and obscurants can prevent the observer from receiving the reflected solar radiation; hence, another spectral region (the 8 to 14 micron, far-infrared, or thermal) is often used to detect targets because ambient temperature radiation from the targets themselves are peaked in the 8 to 14 micron spectral region. In order to assess how well a thermal imaging system will perform for target detection the mechanisms which produce changes in the received thermal image must be quantitatively measured. These mechanisms which produce contrast include: solar radiation which is often variable for partly cloudy conditions, windspeed which cools surfaces convectively or through an evaporative process, precipitation which can rapidly cool warm objects, and a variety of atmospheric obscurants and turbulence which produce attenuation and distortion of the received scene radiation distribution. It is here that the present invention is utilized. The near field/far-field image comparison technique of the Target Contrast Characterizer (described in The Proceedings of the SPIE International Symposium on Optical, Infrared, and Millimeter Wave Propagation Engineering, VOL. 926. Orlando, Fla. [1988]) can be used to separate target contrast change components of interest or closeup target contrast changes from the propagation degradation of the inherent contrast to the distant observation location. To properly quantify the propagation degradation a stable thermal spatial target is needed because the inherent signature of a target against a background can change rapidly and even go through periods of no contrast during the thermal reversals of the diurnal cycle. The target board which is the subject of the invention presented herein can be used to produce very uniform spatial frequency patterns with near perfect transistions between hot and cold portions which do not change during the diurnal cycle and which are not impacted by environmental changes. To be useful such a target board must be large (on the order of two meters by two meters), yet lightweight for ease in transporting over rough terrain to typical target locations, and the spatial patterns must be changeable to meet specific target spatial feature characterization requirements. No prior art thermal test target with a uniform surface temperature is known which can be used to characterize and measure thermal image degradation due to atmospheric propagation of the image radiation field. Nothing currently exists which meets these surface temperature uniformity standards requirements. Those target boards which use other methods such as temperature controlled cooling liquids to create uniform surface temperatures weigh several times more than the thermal target test board, which is the subject of this invention.